1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method of a semiconductor device.
2. Description of the Related Art
Recently, along with downsizing and lowering of operation voltage of field-effect transistors, thinning of gate dielectric films has been desired. Generally, a silicon oxide (SiO2) film is used for a gate dielectric film in field-effect transistors. However, if the SiO2 film is made thinner than 3 nanometers (nm), a leak current due to a tunnel current increases, and it leads to increase in the power consumption. To suppress the increase in the leak current due to the tunnel current, a high dielectric-constant material film (hereinafter, “High-K film”), such as an HfO2 film, capable of achieving a thin SiO2 film having a capacitance-conversion film thickness, although having a large physical film thickness, has come to be used. In this case, an SiO2 film and a silicon oxide-nitride film (an SiON film) are used for an interface layer between the High-K film and a silicon (Si) substrate (for example, see JP-A 2004-289082 (KOKAI)). From the viewpoint of thinning of an entire gate dielectric film, it is preferable that the thickness of the interface layer be as small as possible. However, with the conventional method, as explained in detail below, forming an interface layer having a thickness equal to or smaller than 1 nm was difficult because of reoxidation and nitrogen introduction. For example, in a typical semiconductor-device manufacturing process, cleaning is performed immediately before forming a gate dielectric film. In a liquid solution process used in the cleaning process, however, a chemical oxide film (SiO2) of about 1 nanometer is already formed.
Even when an interface layer including an SiO2 film having a thickness equal to or smaller than 1 nm is first formed by heat oxidation, in a subsequent process of forming the High-K film or in a later process, a silicon substrate interface is reoxidized by active oxygen generated in the High-K film or by active oxygen entered from the outside. As a result, the film thickness of the interface layer increases. To suppress a rate of reoxidation, a method can be considered that uses an SiON film, which is a film that incorporates nitrogen into an SiO2 film, as an interface layer to be formed first. However, a film thickness of the interface layer (the SiON film) also increases in a nitridation process of introducing nitrogen into the interface layer, and the film thickness of the interface finally increases to 1 nm or more. Particularly, when nitrogen concentration in the interface layer at the interface with the silicon substrate is attempted to be increased to suppress the rate of reoxidation, the increase in the film thickness at the time of introducing nitrogen becomes large. Which means that, with the conventional method, it was difficult to suppress the thickness of the interface layer to 1 nm or smaller while suppressing the reoxidation of the interface layer.
Furthermore, when a plasma nitridation process is used to nitrogenize the SiO2 film in the interface layer, it is difficult to efficiently incorporate nitrogen into the interface layer at the silicon substrate side generating the reoxidation, because the plasma nitridation process is a surface reaction process (for example, see JP-A 2004-23008 (KOKAI)).